Chlorine dioxide, which is used in bleaching operations such as the bleaching of cellulosic fibers, may be produced in a variety of manners, generally involving reduction of chlorate by chloride in the presence of an acid. The chlorine dioxide normally is used in the form of an aqueous solution. In as much as chlorine dioxide is of considerable commercial interest and importance in such areas as pulp bleaching, as indicated above, water purification, fat bleaching, removal of phenols from industrial waste, textile bleaching and the like, it is desirable to provide processes by which the chlorine dioxide may be economically produced, and the amount of chlorine produced therewith may be controlled. The basic reaction involved in conventional processes is summarized by the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
Commonly, formation of chlorine dioxide involves the reduction of an alkali metal chlorate with alkali metal chloride in an acid medium. The reactions which occur are exemplified below. For the sake of illustration, the chlorate used is sodium chlorate, the chloride used is sodium chloride, and the strong acid used is sulfuric acid: EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O
An alternative process involves the reduction of the chlorate by hydrochloric acid, the hydrochloric acid providing both the reductant and the acid medium. This process, wherein the alkali metal is sodium, is exemplified by the equation: EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+NaCl
Such reactions are employed commercially, with the reactants continuously fed into a reaction vessel and the chlorine and chlorine dioxide produced therein continuously removed from the reaction vessel.
A single vessel process for producing chlorine dioxide is set forth in U.S. Pat. No. 3,563,702, the teachings of which are hereby incorporated by reference, wherein alkali metal chlorate, an alkali metal chloride and a mineral acid solution are continuously fed to a single vessel generator-evaporator-crystallizer in proportions sufficient to generate chlorine dioxide and chlorine, at a temperature of from about 50.degree. to about 100.degree. centigrade, and an acidity of from about 2 to higher than about 5 normal, in the presence of a catalyst, or at about 4-12 normal without catalyst, removing water by vacuum induced evaporation at about 100-140 millimeters of mercury absolute, with concurrent withdrawal of chlorine dioxide and chlorine, crystallizing the salt of the mineral acid within the generator and withdrawing the crystals from the vessel. Such a system is commercially available, under the trade designation SVP.RTM. Process, from Hooker Chemicals & Plastics Corp.
As the reaction occurs within the generator, in reactions where sulfuric acid is employed as a mineral acid reactant, crystals of sodium sulfate and sodium acid sulfate in amounts and presence dependent generally upon the acid concentration used, are crystallized out and settle to the bottom of the generator from whence they are withdrawn in the form of a slurry.
In addition to the use of sulfuric acid, hydrochloric acid can also be used as the mineral acid reactant, in which instance the crystals removed from the generator are alkali metal chloride crystals, which product is often less desirable than alkali metal sulphate. Sodium sulphate is a valuable by-product, useful in kraft pulping operations, as is the chlorine dioxide. Therefore, systems producing chlorine dioxide and sodium sulfate are particularly useful inasmuch as on-site co-ordination can be effected with pulping operations, utilizing both the primary chlorine dioxide product and the recovered sodium sulfate in the pulping process, particularly in kraft mill operations.
In some instances, however, the requirement for sodium sulfate is greatly reduced or obviated. In certain types of pulping process, sodium sulfate is not required. In certain kraft mill operations, the requirements for sodium sulfate may be reduced or varied, and the disposal of excess salt produces problems, in view of environmental protection standards presently in force. While the requirement for reduced quantities of sodium sulfate may vary, the requirement for the chlorine dioxide remains.
In such instances where reduced quantities or no sodium sulfate is required, the single vessel process can be converted to utilize hydrochloric acid as the mineral acid reactant, in which instance the by-product is sodium chloride. However, such systems are not as efficient as the systems employing sulfuric acid. Further, only sodium chloride is produced and in those instances where varying quantities of sodium sulfate are required, to generate the required amount of sodium sulfate would necessitate the switching back and forth from a catalyzed sulfuric acid system to a catalyzed hydrochloric acid system, with all the problems attendant thereto.
The present invention may be utilized in any conventional chlorine dioxide generating process utilizing a chloride reducing agent, wherein chlorine is concurrently produced. Exemplary systems include the SVP.RTM. II Process, available from Hooker Chemicals & Plastics Corp., as well as the R-2 Process described in U.S. Pat. Nos. 2,863,722, issued Dec. 9, 1958 and 2,936,219 issued May 10, 1960, and the Kesting Process described in Chlorine--Its Manufacture, Properties and Uses, J. S. Sconce, 1967 at page 538. It may be seen that such commercial processes conventionally produce a mixture of chlorine dioxide and chlorine. The advantage of using ClO.sub.2 in place of Cl.sub.2 in such uses as pulp bleaching is that is gives high brightness with little loss in fibrous strength. Presently, all existing commercial processes produce Cl.sub.2 in varying quantities along with the ClO.sub.2. There is a nominal separation in the ClO.sub.2 scrubbers used in such plants, since ClO.sub.2 is much more soluble than Cl.sub.2 in water. The resultant effluent from the scrubber contains about 8 grams per liter ClO.sub.2 and from 1.5 to 2 grams per liter Cl.sub.2. Excess Cl.sub.2 is recovered from the top of the scrubber and fed to a caustic scrubber. However, if excessive Cl.sub.2 is contained in the ClO.sub.2 --H.sub.2 O solution, pulp fiber strength is deleteriously affected. The one to two grams per liter of chlorine presently obtained is marginal from this consideration, and it would be desirable if a more effective separation of ClO.sub.2 from Cl.sub.2 were possible.
Various procedures for selectively removing Cl.sub.2 from ClO.sub.2 and vice versa have been utilized through the years. In 1936, U.S. Pat. No. 2,036,311 taught that oxides, hydroxides, and carbonates of alkaline or alkaline earth metals, in the presence of water, selectively absorbed Cl.sub.2.
U.S. Pat. No. 2,078,045 taught that continued chlorination of calcium oxide resulted in formation of calcium chlorate, which when treated with HCl formed ClO.sub.2.
U.S. Pat. No. 2,108,976 taught that when a ClO.sub.2 --Cl.sub.2 gas mixture was bubbled through aqueous H.sub.2 SO.sub.4, Cl.sub.2 is selectively absorbed, which may be later recovered by air stripping. Similarly, when ClO.sub.2 --Cl.sub.2 is bubbled through dilute aqueous HCl, ClO.sub.2 is selectively absorbed.
U.S. Pat. No. 3,063,218 taught that ClO.sub.2 is selectively absorbed from Cl.sub.2 when contacted with silica gel at temperatures greater than 30.degree. C., forming a stable mixture of chlorine dioxide and silica gel. The chlorine dioxide could be desorbed by increasing the temperature and stripping with air. Rapson et al. taught in U.S. Pat. No. 2,481,241 that ClO.sub.2 could be purified by adding sufficient SO.sub.2 to react with the Cl.sub.2. Rapson also taught countercurrent reaction of ClO.sub.2 --Cl.sub.2 gas mixtures with an equimolar solution of NaClO.sub.3 and NaClO.sub.2, in U.S. Pat. No. 2,871,097.
All of the proceeding processes would yield purified ClO.sub.2. However, none appear to be practical from an operational or economical point of view. There is, at the present, no need to obtain pure ClO.sub.2, nor is there need to eliminate Cl.sub.2 production entirely. It is only necessary to reduce the present one to two grams per liter Cl.sub.2 level in the ClO.sub.2 solution presently obtained. Further, since the pulp mill operations commonly utilize Cl.sub.2 in their bleaching operations, it would seem unnecessary to go to the expense of totally eliminating Cl.sub.2 production.